Some admin page fixes

[dsevillamartin]

Changes proposed in this pull request:

• Ask for confirmation when deleting reactions
  • Since this can delete a lot of people's reactions to posts, thought a confirmation (an additional step) was good to have
• Fix modifying newly-created reaction (https://discuss.flarum.org/d/20655-friendsofflarum-reactions/268)
• Fix fuzzy emoji search breaking when no results are found (https://discuss.flarum.org/d/20655-friendsofflarum-reactions/269)

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).